[Rimbaud]

Like most characters, Gandalf is an amalgamation, taken from a myriad sources. Particularly with Tolkien, I would be hesitant before stating a 'primary' source for any of his characters (although the arguments linking his mother with his often less than three-dimensional female characters is always a semi-amusing debate...)

The Kalevala, yes, Lewis, yes, but many many other sources (eg The Poetic Edda) and original ideas also. Gandalf was once the name of the Chief Dwarf; I think I prefer it the way it worked out...